docs: Add initial MongoDB database schema documentation including ERD and collection details.

Author: HackyCoder0951

docs/DB_Schema.md

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+# Database Documentation
+
+## Overview
+The application uses **MongoDB** as its primary data store. Due to the document-oriented nature of MongoDB, we do not have rigid tables, but we enforce schemas using **Pydantic Models** in the application layer.
+
+## Entity Relationship Diagram (ERD)
+
+Although MongoDB is NoSQL, the entities have implicit relationships.
+
+```mermaid
+erDiagram
+    USERS ||--|{ TASKS : "owns"
+    
+    USERS {
+        ObjectId _id PK
+        string email UK
+        string hashed_password
+        string name
+        enum role "USER | ADMIN"
+        list permissions
+        datetime created_at
+    }
+
+    TASKS {
+        ObjectId _id PK
+        string title
+        string description
+        string owner_id FK "Reference to USERS._id"
+        datetime created_at
+    }
+```
+
+## Collection Details
+
+### 1. Users Collection (`users`)
+Stores all registered user information.
+- **_id**: Automatically generated MongoDB ObjectId.
+- **email**: Unique index ensures no duplicate registrations.
+- **role**: Determines access level. Defaults to `USER`.
+- **permissions**: List of granular permission strings (extensible).
+
+### 2. Tasks Collection (`tasks`)
+Stores to-do items for users.
+- **_id**: Automatically generated MongoDB ObjectId.
+- **owner_id**: A string representation of the User's ObjectId. This links the task to a specific user.
+- **Index**: Typically indexed on `owner_id` for fast retrieval of a user's tasks.
+
+## Implementation Details
+
+### Model Validation
+We use Pydantic models to validate data before it enters the database.
+- **Location**: `backend/app/models/`
+- **Files**: `user.py`, `task.py`
+
+### ID Handling
+MongoDB uses `_id` (ObjectId), but API clients typically expect `id` (string).
+- Our Pydantic models use `Field(alias="_id")` to map the database `_id` to the API `id` field automatically.